The variable pulley transmission, or continuously variable transmission (CVT) as it is commonly called, has been under development for over two decades, but its use as a power transmission is mainly for automotive applications. Its control is rather complicated and is very sensitive to operating conditions. For example, in a control arrangement for a variable pulley transmission disclosed in U.S. Pat. No. 4,458,318, a variable line pressure is controlled and applied to the sheave of the secondary or driven pulley of the transmission, and also serves as the hydraulic power supply to a ratio control valve in a speed control loop of the transmission.
Applicant has found that due to the variable supply pressure in the continuously variable transmission of U.S. Pat. No. 4,458,318, both the pressure control valve (a pressure relief type valve) and the ratio control valve gains change with the supply pressure, and the control is therefore very sensitive to system operating conditions. There is a need for an improved continuously variable transmission with a control arrangement and method for reduction of belt slippage in a continuously variable transmission which overcome these drawbacks and disadvantages of this known continuously variable transmission. More particularly, there is a need for an improved continuously variable transmission with a control arrangement and method which are simpler and more robust than those disclosed in U.S. Pat. No. 4,458,318. It is an object of the present invention to provide a continuously variable transmission with a control arrangement and method which meet this need.
Another object of the present invention is to provide a continuously variable transmission with a control arrangement and method for use as a constant speed drive for an aircraft electrical power generator wherein belt slippage can be minimized.
These and other objects are attained by the continuously variable transmission of the present invention for transferring drive from an engine to a device to be driven wherein the transmission comprises a primary pulley for receiving drive from an engine, a belt, a secondary pulley which is coupled over the belt to the primary pulley for transferring drive to a device to be driven, each of the primary and secondary pulleys having an axially movable sheave and a hydraulically operated actuator therefor to effect ratio change of the transmission and to maintain belt tension, and wherein a single source of constant hydraulic pressure is operatively connected for driving the actuators. In the disclosed form of the invention, the source of constant hydraulic pressure comprises a hydraulic pump and a pressure relief valve in communication with the output of the hydraulic pump to maintain a constant predetermined hydraulic output pressure of the pump. The single source of constant hydraulic pressure is operatively connected to the actuator of the primary pulley by way of a ratio control valve and to the actuator of the secondary pulley by way of a pressure control valve.
According to further features of the invention, the continuously variable transmission includes a hydraulic pressure control loop for controlling the hydraulic pressure applied to the actuator of the secondary pulley as a function of the sensed load of the device to be driven on the transmission and the pitch radius of the secondary pulley. An output speed control loop is also provided for controlling the output speed of the transmission driving the device to be driven. In the disclosed embodiment the output speed control loop may be set to control the output speed at a constant value for driving an aircraft electric generator. According to another feature of the invention, the hydraulic pressure control loop measures the speed of the generator and the load current of the generator being driven for calculation of the load torque of the generator on the transmission.
A method of the invention for reducing belt slippage in the continuously variable transmission comprises determining the hydraulic pressure to be applied by the hydraulic pressure control loop to the actuator of the secondary pulley to keep the belt from slipping. This determining is accomplished according to the disclosed embodiment by determining the load torque transmitted from the device to be driven to the secondary pulley of the transmission and the pitch radius of the secondary pulley; and calculating a value for the set pressure to be applied by the hydraulic pressure control loop to the actuator of the secondary pulley in accordance with a relationship disclosed hereinafter utilizing the determined load torque and pitch radius of the secondary pulley together with the coefficient of friction between the belt and the secondary pulley and the area of the actuator for the axially movable sheave of the secondary pulley. The calculated value for the set pressure is preferably multiplied by a factor slightly greater than 1, such as 1.2, to determine a set pressure of the hydraulic pressure control loop to be applied to the actuator of the secondary pulley with some margin for safety against belt slippage while guarding against use of too large a pressure which would shorten the life of the belt and pulleys.
These and other objects, features and advantages of the present invention will become more apparent from the following detailed description of the invention taken with the accompanying drawings depicting a preferred embodiment in accordance with the invention.